package TestDemo;

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int m = sc.nextInt();
        int[] step = new int[m + 1];
        for(int i = 0; i < m + 1; i++){
            step[i] = Integer.MAX_VALUE;
        }
        step[n] = 0;
        for(int i = n; i < m; i++){
            if(step[i] == Integer.MAX_VALUE){
                continue;
            }
            List<Integer> list = div(i);
            for(int j : list){
                if(i + j <= m && step[i + j] != Integer.MAX_VALUE){
                    step[i + j] = Math.min(step[i + j], step[i] + 1);
                }else if(i + j <= m){
                    step[i + j] = step[i] + 1;
                }
            }
        }
        if(step[m] == Integer.MAX_VALUE){
            System.out.println(-1);
        } else {
            System.out.println(step[m]);
        }
    }
    public static List<Integer> div(int num){
        List<Integer> list = new ArrayList<>();
        for(int i = 2; i * i <= num; i++){
            if(num % i == 0){
                list.add(i);
                if(num / i != i){
                    list.add(num / i);
                }
            }
        }
        return list;
    }
}